The present invention relates to using CCD (charge coupled device) imagers in a television camera, and more particularly, to using such imagers that have defects.
Present CCD imagers suffer from low yields due to imperfections and defects in the integrated circuit chips from which they are fabricated. Various schemes have been used to correct for the signal disturbances that result from such defects. For example U.S. Pat. No. 3,904,818 shows a system that in effect does dropout compensation. The system detects when a photosensor provides excessive dark current, and if so, substitutes a signal that is the average of signals provided by photosensors surrounding the defective one. However, an average of signals from surrounding photosensors may not be a close enough approximation to be satisfactory. Also, other photosensors in the area may be affected by the defect and averaging may not produce an acceptable correction.
Other schemes, such as shown in U.S. patent application Ser. No. 242,265, filed Mar. 10, 1981, in the names of W. H. Meise and R. A. Dischert, use two imagers and two ROMs (read only memories) programmed with defect locations of the imagers.
Normally the output signal is derived from both imagers for best signal-to-noise ratio. When a defective photosensor is about to be read from one imager, the output signal is derived from just the other imager. However, this scheme may require complex and expensive circuitry, e.g. the ROMs and their associated circuits.
Still another scheme is shown in Japanese Patent (Kokai) No. 54-56722. As shown therein a flaw detector, which comprises a threshold circuit, receives the output signal from one of two imagers. When the output signal exceeds the peak white level or goes below black level, thereby indicating a defective photosensor, the threshold circuit provides an output signal to perform a switching function so that the video signal output for the entire circuit is now derived from the second imager. However, this circuit cannot detect grey level defects that result in incorrect video signals between the black and white levels.
It is therefore desirable to have a defect compensation circuit that has high resolution, low cost, and detects grey level as well as severe defects.